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The telomere is the specialized nucleoprotein complex at the end of the chromosome. Its highly conserved 5′‐TTAGGG‐3′ repeats and shelterin protein complexes form a protective loop structure to maintain the integrity and stability of linear chromosomes. Although human somatic cells gradually shorten telomeres to undergo senescence or crisis, cancer cells activate telomerase, or the recombination‐based mechanism to maintain telomeres and exhibit immortality. As the most frequent non‐coding mutations in cancer, gain‐of‐function mutations in the promoter region of the telomerase catalytic subunit, TERT, trigger telomerase activation. Promoter methylation and copy number gain are also associated with the enhanced TERT expression. Although telomerase inhibitors were pioneered from telomere‐directed therapeutics, their efficacies are limited to cancer with short telomeres and some hematological malignancies. Other therapeutic approaches include a nucleoside analog incorporated to telomeres and TERT promoter‐driven oncolytic adenoviruses. Tankyrase poly(ADP‐ribose) polymerase, a positive regulator of telomerase, has been rediscovered as a target for Wnt‐driven cancer. Meanwhile, telomeric nucleic acids form a higher‐order structure called a G‐quadruplex (G4). G4s are formed genome‐wide and their dynamics affect various events, including replication, transcription, and translation. G4‐stabilizing compounds (G4 ligands) exert anticancer effects and are in clinical investigations. Collectively, telomere biology has provided clues for deeper understanding of cancer, which expands opportunities to discover innovative anticancer drugs. Starting from the chromosome ends, telomeres, and their functional modulators have brought new facets to our strategies for anticancer drug discovery. Promising approaches beyond targeting cell immortality include TERT/telomerase hijacking, Wnt signal inhibition, and stabilization of G‐quadruplexes.

Telomeres, the protective structures of chromosome ends are gradually shortened by each cell division, eventually leading to senescence or apoptosis. Cancer cells maintain the telomere length for unlimited growth by telomerase reactivation or a recombination-based mechanism. Recent genome-wide analyses have unveiled genetic and epigenetic alterations of the telomere maintenance machinery in cancer. While telomerase inhibition reveals that longer telomeres are more advantageous for cell survival, cancer cells often have paradoxically shorter telomeres compared with those found in the normal tissues. In this review, we summarize the latest knowledge about telomere length alterations in cancer and revisit its rationality. Finally, we discuss the potential utility of telomere length as a prognostic biomarker.

Aberrant activation of Wnt/β‐catenin signaling causes tumorigenesis and promotes the proliferation of colorectal cancer cells. Porcupine inhibitors, which block secretion of Wnt ligands, may have only limited clinical impact for the treatment of colorectal cancer, because most colorectal cancer is caused by loss‐of‐function mutations of the tumor suppressor adenomatous polyposis coli (APC) downstream of Wnt ligands. Tankyrase poly(ADP‐ribosyl)ates (PARylates) Axin, a negative regulator of β‐catenin. This post‐translational modification causes ubiquitin‐dependent degradation of Axin, resulting in β‐catenin accumulation. Tankyrase inhibitors downregulate β‐catenin and suppress the growth of APC‐mutated colorectal cancer cells. Herein, we report a novel tankyrase‐specific inhibitor RK‐287107, which inhibits tankyrase‐1 and ‐2 four‐ and eight‐fold more potently, respectively, than G007‐LK, a tankyrase inhibitor that has been previously reported as effective in mouse xenograft models. RK‐287107 causes Axin2 accumulation and downregulates β‐catenin, T‐cell factor/lymphoid enhancer factor reporter activity and the target gene expression in colorectal cancer cells harboring the shortly truncated APC mutations. Consistently, RK‐287107 inhibits the growth of APC‐mutated (β‐catenin‐dependent) colorectal cancer COLO‐320DM and SW403 cells but not the APC‐wild (β‐catenin‐independent) colorectal cancer RKO cells. Intraperitoneal or oral administration of RK‐287107 suppresses COLO‐320DM tumor growth in NOD‐SCID mice. Rates of tumor growth inhibition showed good correlation with the behavior of pharmacodynamic biomarkers, such as Axin2 accumulation and MYC downregulation. These observations indicate that RK‐287107 exerts a proof‐of‐concept antitumor effect, and thus may have potential for tankyrase‐directed molecular cancer therapy. Aberrant activation of Wnt/β‐catenin signaling causes tumorigenesis and promotes the proliferation of colorectal cancer cells. We report a novel tankyrase‐specific PARP inhibitor RK‐287107, which inhibits tankyrase‐mediated PARylation of Axin and downregulates β‐catenin signaling in APC‐mutated colorectal cancer cells. RK‐287107 exerts a proof‐of‐concept antitumor effect in vivo, and thus may have potential for cancer therapy.

G-quadruplexes (G4s) are higher-order structures formed by guanine-rich sequences of nucleic acids, such as the telomeric 5′-TTAGGG-3′/5′-UUAGGG-3′ repeats and those in gene regulatory regions. G4s regulate various biological events, including replication, transcription, and translation. Imbalanced G4 dynamics is associated with diseases, such as cancer and neurodegenerative diseases. Telomestatin is a natural macrocyclic compound derived from Streptomyces anulatus 3533-SV4. It interacts with the guanine quartet via π-π stacking and potently stabilizes G4. Because G4 stabilization at the telomeric repeat inhibits the telomere-synthesizing enzyme telomerase, telomestatin was originally identified as a telomerase inhibitor. Whereas non-toxic doses of telomestatin induce gradual shortening of telomeres and eventual crisis in human cancer cells, higher doses trigger prompt replication stress and DNA damage responses, resulting in acute cell death. Suppression of the transcription and translation of G4-containing genes is also implicated in the anticancer effects of telomestatin. Because telomestatin is rare, labile, and insoluble, synthetic oxazole telomestatin derivatives have been developed and verified for their therapeutic efficacies in preclinical cancer models. Furthermore, a variety of G4-stabilizing compounds have been reported as promising seeds for molecular cancer therapeutics. To improve the design of future clinical studies, it will be important to identify predictive biomarkers of drug efficacy.

Tumors consist of heterogeneous cell populations that contain cancer cell subpopulations with anticancer drug‐resistant properties called “persister” cells. While this early‐phase drug tolerance is known to be related to the stem cell‐like characteristic of persister cells, how the stem cell‐related pathways contribute to drug resistance has remained elusive. Here, we conducted a single‐cell analysis based on the stem cell lineage‐related and gastric cell lineage‐related gene expression in patient‐derived gastric cancer cell models. The analyses revealed that 5‐fluorouracil (5‐FU) induces a dynamic change in the cell heterogeneity. In particular, cells highly expressing stem cell‐related genes were enriched in the residual cancer cells after 5‐FU treatment. Subsequent functional screening identified aldehyde dehydrogenase 1A3 (ALDH1A3) as a specific marker and potential therapeutic target of persister cells. ALDH1A3 was selectively overexpressed among the ALDH isozymes after treatment with 5‐FU or SN38, a DNA topoisomerase I inhibitor. Attenuation of ALDH1A3 expression by RNA interference significantly suppressed cell proliferation, reduced the number of persister cells after anticancer drug treatment and interfered with tumor growth in a mouse xenograft model. Mechanistically, ALDH1A3 depletion affected gene expression of the mammalian target of rapamycin (mTOR) cell survival pathway, which coincided with a decrease in the activating phosphorylation of S6 kinase. Temsirolimus, an mTOR inhibitor, reduced the number of 5FU‐tolerant persister cells. High ALDH1A3 expression correlated with worse prognosis of gastric cancer patients. These observations indicate that the ALDH1A3‐mTOR axis could be a novel therapeutic target to eradicate drug‐tolerant gastric cancer cells. Using a single‐cell analysis of patient‐derived gastric cancer cells, we identified aldehyde dehydrogenase 1A3 (ALDH1A3) as a therapeutic target of anticancer drug‐tolerant persister cells. ALDH1A3 knockdown significantly suppressed cell proliferation and reduced the number of persister cells after treatment with 5‐fluorouracil (5‐FU) and SN38. Mechanistically, ALDH1A3 depletion downregulated the mTOR cell survival pathway, and an mTOR inhibitor reduced the number of drug‐tolerant persister cells.

Numerous epithelial–mesenchymal transition (EMT) characteristics have now been demonstrated to participate in tumor development. Indeed, EMT is involved in invasion, acquisition of stem cell properties, and therapy‐associated resistance of cancer cells. Together, these mechanisms offer advantages in adapting to changes in the tumor microenvironment. However, recent findings have shown that EMT‐associated transcription factors (EMT‐TFs) may also be involved in DNA repair. A better understanding of the coordination between the DNA repair pathways and the role played by some EMT‐TFs in the DNA damage response (DDR) should pave the way for new treatments targeting tumor‐specific molecular vulnerabilities, which result in selective destruction of cancer cells. Here we review recent advances, providing novel insights into the role of EMT in the DDR and repair pathways, with a particular focus on the influence of EMT on cellular sensitivity to damage, as well as the implications of these relationships for improving the efficacy of cancer treatments. Novel insights into the role of epithelial to mesenchymal transition in the DNA damage response and repair pathways, as well as the implications of these relationships for improving the efficacy of cancer treatments.

Cancer cells adopt multiple strategies to escape tumor surveillance by the host immune system and aberrant amino acid metabolism in the tumor microenvironment suppresses the immune system. Among the amino acid‐metabolizing enzymes is an L‐amino‐acid oxidase called interleukin‐4 induced 1 (IL4I1), which depletes essential amino acids in immune cells and is associated with a poor prognosis in various cancer types. Although IL4I1 is involved in immune metabolism abnormalities, its effect on the therapeutic efficacy of immune checkpoint inhibitors is unknown. In this study, we established murine melanoma cells overexpressing IL4I1 and investigated their effects on the intratumor immune microenvironment and the antitumor efficacy of anti‐programmed death‐ligand 1 (PD‐L1) antibodies (Abs) in a syngeneic mouse model. As a result, we found that IL4I1‐overexpressing B16‐F10‐derived tumors showed resistance to anti‐PD‐L1 Ab therapy. Transcriptome analysis revealed that immunosuppressive genes were globally upregulated in the IL4I1‐overexpressing tumors. Consistently, we showed that IL4I1‐overexpressing tumors exhibited an altered subset of lymphoid cells and particularly significant suppression of cytotoxic T cell infiltration compared to mock‐infected B16‐F10‐derived tumors. After treatment with anti‐PD‐L1 Abs, we also found a more prominent elevation of tumor‐associated macrophage (TAM) marker, CD68, in the IL4I1‐overexpressing tumors than in the mock tumors. Consistently, we confirmed an enhanced TAM infiltration in the IL4I1‐overexpressing tumors and a functional involvement of TAMs in the tumor growth. These observations indicate that IL4I1 reprograms the tumor microenvironment into an immunosuppressive state and thereby confers resistance to anti‐PD‐L1 Abs. Interleukin‐4 induced 1 overexpression upregulates immunosuppressive gene expression, decreases CD8+ T cell infiltration, and causes resistance to the anti‐ programmed death‐ligand‐1 Ab therapy in a mouse melanoma model.

The vascular endothelial growth factor (VEGF)/VEGF receptor (VEGFR) axis is an essential regulator of angiogenesis and important therapeutic target in cancer. Ramucirumab is an anti-VEGFR2 monoclonal antibody used for the treatment of several cancers. Increased circulating VEGF-A levels after ramucirumab administration are associated with a worse prognosis, suggesting that excess VEGF-A induced by ramucirumab negatively affects treatment efficacy and that neutralizing VEGF-A may improve treatment outcomes. Here, we evaluated the effect of combination treatment with an anti-VEGFR2 antibody and anti-VEGF-A antibody on gastric tumor progression and normal tissues using a preclinical BALB/c-nu/nu mouse xenograft model. After anti-VEGFR2 antibody treatment in mice, a significant increase in plasma VEGF-A levels was observed, mirroring the clinical response. The elevated VEGF-A was host-derived. Anti-VEGF-A antibody co-administration enhanced the anti-tumor effect of the anti-VEGFR2-antibody without exacerbating the toxicity. Mechanistically, the combination treatment induced intra-tumor molecular changes closely related to angiogenesis inhibition and abolished the gene expression changes specifically induced by anti-VEGFR2 antibody treatment alone. We particularly identified the dual treatment-selective downregulation of ZEB1 expression, which was critical for gastric cancer cell proliferation. These data indicate that the dual blockade of VEGF-A and VEGFR2 is a rational strategy to ensure the anti-tumor effect of angiogenesis-targeting therapy.

Cellular senescence causes a dramatic alteration of chromatin organization and changes the gene expression profile of proinflammatory factors, thereby contributing to various age-related pathologies through the senescence-associated secretory phenotype (SASP). Chromatin organization and global gene expression are maintained by the CCCTC-binding factor (CTCF); however, the molecular mechanism underlying CTCF regulation and its association with SASP gene expression remains unclear. We discovered that noncoding RNA (ncRNA) derived from normally silenced pericentromeric repetitive sequences directly impairs the DNA binding of CTCF. This CTCF disturbance increases the accessibility of chromatin and activates the transcription of SASP-like inflammatory genes, promoting malignant transformation. Notably, pericentromeric ncRNA was transferred into surrounding cells via small extracellular vesicles acting as a tumorigenic SASP factor. Because CTCF blocks the expression of pericentromeric ncRNA in young cells, the down-regulation of CTCF during cellular senescence triggers the up-regulation of this ncRNA and SASP-related inflammatory gene expression. In this study, we show that pericentromeric ncRNA provokes chromosomal alteration by inhibiting CTCF, leading to a SASP-like inflammatory response in a cell-autonomous and non–cell-autonomous manner and thus may contribute to the risk of tumorigenesis during aging.